Apparatus:
Concave mirror of long focal length vertical stand with clamp, a plumb line/parallax needle. Meter rod, index needle, liquid (water) and wooden stand)
Observations:
Approximate focal length of a concave mirror = ………………cm
| No.ofObs. | Distance between the pole of mirror and the tip of the needle in no parallax position | Refractive index | |||
| Without liquid | AP | With liquid | A′P | ||
| 1234 | |||||
Mean refractive Index ==…………….
Procedure:
Find the approximate focal length of a concave mirror as before. Place it in a flat wooden stand with principle axis vertical. Clamp the parallax needle horizontally in the stand and placing it above the mirror at about the approximate focal length. Remove the parallax between tip of parallax needle and its inverted image. In no parallax position. the needle is at the center of curvature of the mirror. With the help of meter rod and plumb line, measure the distance AP between the tip of the needle and the pole of the mirror. Now put a few drops of liquid (water) which forms the thin layer in the mirror the mirror. This layer of water disturbs the no parallax position. In order to remove the parallax again, lower the needle and adjust it such that it’s no parallax position is reached. Measure the distance AʹP between the pole of the mirror and the tip of the parallax needle. Repeat the experiment twice and find (refractive index) each and take its mean value.
Precautions:
1. Parallax needle should be well illuminated.
2. Mirror should be horizontal and its principal axis vertical.
- A small quantity should be used to get a thin layer on the surface of the mirror.
4. Distance should be measure very carefully.
5. Use a concave mirror of large focal length.
VIVA VOCE
Q.1. What is parallax?
Ans. The separation between object and image needle caused by relative motion of eye is called parallax.
- Define radius of curvature of spherical mirror?
Ans. Radius of curvature of spherical mirror is the radius of sphere of which mirror is a part.
- Why should we use a concave mirror of large radius of curvature?
Ans. To justify the approximation adopted in the formula.
- What is an optically denser medium?
Ans. A medium in which light travels slower it moves that in air, is called an optically denser medium.
- Where does the image appear to be when parallax to remove with water?
Ans. It appears to be situated at the center of curvature of the mirror. Actually the rays retrace them
path as they fall along the radius of curvature of the mirror after refraction.
- What would happen if a very small quantity of liquid to used?
Ans. The surface of the liquid will not be perfectly flat and the lens effect will produce distortion.
- Why do we measure distances upto the surface of water for both the positions of the needle?
Ans. For small angles sines are equal to tangents. With the assumption distances need only be measured upto the surface of water.
- What kind of mirror will you select for the experiment?
Ans. A concave mirror of large focal length.